Resin-impregnated fiberglass sheets are commonly used in the formation of printed circuit boards. The fiberglass cloth is typically impregnated with the selected thermoset resin which is then partially cured and the impregnated cloth sheared to form what are known as sticker or prepreg sheets. In order to enhance the adhesion of the resin to the fiberglass, often a coupling agent, such as a silane, is coated onto the surface of the fiberglass prior to impregnation. The prepreg sheets are then laid up with sheets of metal such as copper or copper-invar-copper (CIC) and laminated with heat and pressure to fully cure the laid-up laminate with the metal sheets defining ground, power and signal planes. One of the desirable characteristics of the resin-impregnated fiberglass sheets is that the resin-impregnation must cover the fibers of the fiberglass and must be able to be partially cured to a non-tacky state wherein the sheets can be handled for the lamination process. This is often referred to as a B-stage, a cure state which allows the sheets to be sufficiently self-supporting to be laid up as a laminate, but not advanced enough in the state of cure that they are rigid or non-flowable when heated, and they can be further cured to a final cure with heat and pressure to form a laminate structure as is well known in the art. As indicated above, this lamination process normally includes the lamination of one or more sheets of metal, such as copper, CIC or other metal, to provide necessary ground planes, power planes and signal planes buried within the laminated circuit board. Also in conventional practice, openings are formed, either by drilling or other means, through the fully cured laminates which form the openings for vias or plated through holes where the connections can be made from one surface of the circuit board to the other and to the various internal planes within the laminate as required.
A conventional technique of forming the resin-impregnated fiberglass sheets is to provide a coil of the fiberglass material and unwind the fiberglass material from the coil and continuously pass it through a tank containing the solution of the desired resin in a solvent, and then pass the coated or impregnated material through a treater tower wherein heat is applied to drive off the solvent and to partially cure the resin material by initiating cross-linking and then coiling the partially-cured or B staged material into a coil. Thereafter, the partially-cured material is uncoiled and cut into sheets of the desired length. These sheets, known as prepreg sheets, are then used in the lamination process described above.
This prepreg material has long been used for manufacturing circuit boards, however, more recently, the same prepreg material and same laminating techniques that have been used to form a circuit board have been used to form chip carriers. A chip carrier is basically a small-size version of a circuit board where the metallurgy and the lay out can be much finer than on a circuit board. Printed circuit board reliability tests are defined, e.g., in IPC specifications, whereas chip carrier tests are defined by JEDEC specifications which are derived for ceramic carriers and are more severe tests. In addition, because of the finer geometry of the metallurgy and the lay-out, chip carriers are more prone to failure from various failure mechanisms.
In forming laminate structures which include resin impregnated glass fibers and metal sheets, such as copper, in the laminate, it is known that certain resins adhere better to the glass fibers, and certain resins adhere better to the metal. Thus, there has been a trade-off as to whether to select a resin for good adhesion to the glass fibers to minimize defects that can occur because of poor adhesion to glass fibers, or to select a resin which has better adhesion to the metal and thus reduce the defects of a laminate structure caused by delamination of the metal from the resin-impregnated fiberglass. Indeed, in some instances, a compromise is attempted by mixing two resins, one having better adhesion to glass fibers and the other having better adhesion to metal. This results in a compromise in that neither the adhesion to the glass fiber nor the adhesion to the metal is optimized; and, moreover, this can cause problems by requiring the selection of resins that are compatible with each other and which can be mixed and will coat properly. Even when these problems are overcome, there is still the problem that neither the adhesion to the fiberglass nor the adhesion to the metal is optimized.
Therefore, it is a principal object of the present invention to provide a resin configuration that optimizes adhesion to both the glass fibers and the metal in a laminate.